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Date:   Fri, 5 May 2023 15:19:03 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Tim Chen <tim.c.chen@linux.intel.com>
Cc:     Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Ricardo Neri <ricardo.neri@intel.com>,
        "Ravi V . Shankar" <ravi.v.shankar@intel.com>,
        Ben Segall <bsegall@google.com>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Len Brown <len.brown@intel.com>, Mel Gorman <mgorman@suse.de>,
        "Rafael J . Wysocki" <rafael.j.wysocki@intel.com>,
        Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Valentin Schneider <vschneid@redhat.com>,
        Ionela Voinescu <ionela.voinescu@arm.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org,
        Shrikanth Hegde <sshegde@linux.vnet.ibm.com>,
        Srikar Dronamraju <srikar@linux.vnet.ibm.com>,
        naveen.n.rao@linux.vnet.ibm.com,
        Yicong Yang <yangyicong@hisilicon.com>,
        Barry Song <v-songbaohua@oppo.com>,
        Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
Subject: Re: [PATCH 3/6] sched/fair: Fix busiest group selection for asym
 groups
Message-ID: <20230505131903.GP83892@hirez.programming.kicks-ass.net>
References: <cover.1683156492.git.tim.c.chen@linux.intel.com>
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On Thu, May 04, 2023 at 09:09:53AM -0700, Tim Chen wrote:
> From: Tim C Chen <tim.c.chen@linux.intel.com>
> 
> For two groups that have spare capacity and are partially busy, the
> busier group should be the group with pure CPUs rather than the group
> with SMT CPUs.  We do not want to make the SMT group the busiest one to
> pull task off the SMT and makes the whole core empty.
> 
> Otherwise suppose in the search for busiest group,
> we first encounter an SMT group with 1 task and set it as the busiest.
> The local group is an atom cluster with 1 task and we then encounter an atom
> cluster group with 3 tasks, we will not pick this atom cluster group over the
> SMT group, even though we should.  As a result, we do not load balance
> the busier Atom cluster (with 3 tasks) towards the local Atom cluster
> (with 1 task).  And it doesn't make sense to pick the 1 task SMT group
> as the busier group as we also should not pull task off the SMT towards
> the 1 task atom cluster and make the SMT core completely empty.
> 
> Fix this case.
> 
> Reviewed-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
> Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
> ---
>  kernel/sched/fair.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index bde962aa160a..8a325db34b02 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -9548,6 +9548,18 @@ static bool update_sd_pick_busiest(struct lb_env *env,
>  		break;
>  
>  	case group_has_spare:
> +		/*
> +		 * Do not pick sg with SMT CPUs over sg with pure CPUs,
> +		 * as we do not want to pull task off half empty SMT core
> +		 * and make the core idle.
> +		 */

Comment says what the code does; not why.

> +		if (asymmetric_groups(sds->busiest, sg)) {
> +			if (sds->busiest->flags & SD_SHARE_CPUCAPACITY)
> +				return true;
> +			else
> +				return false;

			return (sds->busiest->flags & SD_SHARE_CPUCAPACITY)
> +		}

Also, should this not be part of the previous patch?